1. Field
The present invention relates to an optical power limiter device for protecting thermal sensors against threat incoming radiation, and especially to providing a plurality of optical power limiters in tandem between window substrates with each limiter having a progressively lower switching threshold temperature to the incoming radiation so that the last limiter is usually the first to switch on by increasing temperature and reflect back the radiation for a second pass through the other limiters and quickly raise their temperatures to threshold level in which the tandem of limiters essentially switch on sequentially to provide increase in optical density for the device.
2. Prior Art
It is highly desirable to protect sensitive thermal sensors against threat laser radiation which disables the performance of the sensors by jamming or damaging components. Some of the requirements for optical power limiter device are that it be passive and self-activated, broadband between say 7 um to 12 um, a field of view of more than 20°, low thermal switching threshold, large optical density in the switched state, and highly transmissive in the unswitched state.
Devices exist that meet some of these requirements. For example, a chalcogenide optical power limiter or a vanadium dioxide (VO2) optical power limiter used separately can meet some of these requirements, but not all of them. The chalcogenide device has a higher switching threshold, a lower damage threshold, and a lower transmission than the requirements. The VO2 device has a low switching threshold, but also has the undesirable low damage threshold and low transmission. Germanium optical power limiters undergo thermal runaway starting at about 75° C. where the absorption coefficient increases rapidly.
Combinations of the above noted optical power limiter materials as in the present device results in an acceptable device with increased damage threshold, improved switching threshold, and improved optical density which protect the sensors and other optical components more effectively.